New Scientist magazine said Wednesday a British study showed that people with conditions such as cancer or HIV were at risk of getting more than just a gleaming smile.

James Walker and his team at the Center for Applied Microbiology and Research found that levels of "opportunistic pathogens" exceeded European Union limits in 52 of 55 water samples taken at 21 dental surgeries in southwestern England.

The research was originally reported in the journal Applied and Environmental Microbiology.

"People with compromised immune systems should worry," David Turner of the British Dental Association told New Scientist.

The bacteria found in the dental unit water lines (DUWs) included species of Mycobacterium and Legionella which can both cause life-threatening pneumonia.

Walker's team also isolated oral streptoccoci -- potentially deadly bacteria which can cause scarlet fever and pneumonia -- in 10 percent of the samples.

"Since the bacteria is only found in the mouth, it is most likely that during dental procedures it was sucked back into the tools and into the DUW," New Scientist said.

Some of the highest bacterial counts were found in DUWs which had been recently sanitized or supplied with bottled water, casting doubt on recommendations for reducing risk put out by the dental associations in Britain and the United States, it added.

"This is like sharing spit," Robert Staat, of the University of Louisville's school of dentistry in the United States, said.

New Scientist said it was told by the American Dental Association that similar studies in several U.S. cities had revealed bacterial counts beyond safety guidelines.

Most of the bacteria in DUWs occur naturally and pose little threat to people with healthy immune systems.

But Hugh Pennington, a microbiologist at the University of Aberdeen in Scotland, said no one visiting the dentist should be exposed to the levels discovered in Walker's study.

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